1. Field of the Invention
This invention relates to a wet friction material that generates a torque by applying a high pressure to an opposite surface while being immersed in an oil. In detail, this invention relates to a segment-type friction material made by joining segment pieces made of a friction material substrate on one or both of opposite surfaces of a core plate or a core plate of a flat ring shape along an entire circumference thereof. Each of the friction material substrates is cut into a segment piece shape. Particularly, this invention relates to a segment-type friction material that enables yield to be improved while making heat-resistance better.
2. Description of the Related Art
In recent year, a segment-type friction material as a wet friction material is under development in order to improve material yield with resultant cost reduction and to lessen drag torque with resultant high fuel efficiency in a vehicle. The segment-type friction material has a core plate of a flat ring shape and segment pieces. The segment pieces are made by cutting a friction material substrate along the ring shape or a circumferential direction of the core plate. A set of the segment pieces are arranged and joined with an adhesive on a front surface of the core plate side by side along an entire circumference of the core plate with a gap to be an oil groove. Another set of the segment pieces made by cutting the friction material substrate are joined with the adhesive to a rear surface of the core plate, too. Such segment-type friction material is applicable to a friction material clutch device that has a single or plural friction plates and that is used in an automatic transmission, which may be referred to as “AT” hereafter, of automobiles or a transmission of motorcycles or the like.
A ring-type friction material is used at another portion of the automatic transmission. The ring-type friction material is made by cutting a friction material substrate into a ring shape and sticking it to a core plate of a flat ring plate shape by an adhesive. However, there is a problem that the ring-type friction material has very low yield in the friction material substrate. Therefore, it is proposed to make very small an interval between the segment pieces of the segment-type friction material or remove the interval thereof. In this case, it is guessed that the segment-type friction material has a similar friction property to that of the ring-type friction material.
Thereby, it is expected that the segment-type friction material shows similar characteristics to that of the ring-type friction material in supplying an automatic transmission fluid (may be referred to as simply “ATF” hereafter). However, in a conventional manufacturing apparatus, there is still a problem that it is impossible to arrange and stick the segment piece on the core metal without an interval between the adjacent segment pieces.
In view of the above, the inventors provide an improved segment-type friction material and its manufacturing method as shown in Japanese Laid Open Patent Publication No. 2005-299731. In this segment-type friction material, the interval between the adjacent segment pieces is made very small or removed. Thereby, the segment-type friction material is given a similar friction property to that of the ring-type friction material, while reducing costs.
Still, the segment-type friction material is required to have a variety of friction characteristics according to use. In order to satisfy various requirements, the segment-type friction material is manufactured such that an interval is provided between the segment pieces as an oil groove. Then, the manufactured segment-type friction material makes a number or position of the oil grooves changed in different manners, thereby satisfying a required frictional property. Moreover, in case of lessening the number of the oil grooves, segment pieces are cut out into a long size or into a wide angled arc. Then, a few number of the long segment pieces are stuck on the core metal so as to cope with the requirement.
In punching out the segment pieces from a band-shaped friction material substrate, a radius of an inner circumference of the segment piece is set equal to a radius of an outer circumference thereof. This is to enable the segment pieces to be cut out successively from an upper side to a lower side (in a longitudinal direction) of the band-shaped friction material substrate. With such cutting operation, no unused portion is remained between the inner circumference and the outer circumference of the adjacent punched-out segment pieces. However, in case of cutting out the long-sized or wide-angled segment pieces, there is a disadvantage that a size in a direction perpendicular to a circumferential direction of the segment piece becomes shorter toward circumferentially opposite ends. Consequently, the yield of the friction material substrate is lowered and a lining area of the segment piece becomes small.
In addition, the segment pieces are to be cut out or punched out into different shapes according to a required position and a required number of the oil grooves for satisfying a certain frictional property. For example, some oil grooves may be arranged to each other at a large distance or at a large angle in one area of the segment-type friction material, while other oil grooves being arranged to each other at a short or medium distance or at a small or medium angle in another area thereof. In this case, it is necessary to prepare two or three kinds (long, medium, short) of segment pieces having a different circumferential length or a different arc angle. That is, one of the segment pieces should have a long size or a wide arc angle so as to be disposed in the one area where the oil grooves are positioned at the large distance. The other of the segment pieces should have a short or medium size or a small or medium arc angle so as to be disposed in the other area where the oil grooves are positioned at the short or medium distance. Therefore, in this case, it is necessary to design and make two or three types of cutting edges for punching out the two kinds of segment piece having different sizes, respectively, in accordance with the size difference (long, medium, short) of the segment pieces. Moreover, the cut-out segment pieces are arranged on the flat ring-shaped core metal, respectively, while rotating the core metal at a prescribed angle by a core metal rotating unit. However, in this case, the core metal rotating unit should be designed and fabricated such that it rotates the core metal at different two or three intervals in accordance with the size difference of the segment pieces. As described above, it is indispensable to design and form different cutting edges and different core metal rotating units in order to cutting out different shapes of segment pieces and to rotate the core metal at different angles for adhesion of the segment pieces. Such design and manufacture may increase costs and make a manufacturing efficiency worse.
The above description relating to “number and position of the oil grooves to obtain a desired frictional property” is not a description as prior art. It is explained herein just as a technique related to an object of the present invention. The applicants do not admit the description as prior art.